Such communication units may form part of a device used in the control of a fuel system of an internal combustion engine in response to monitoring exhaust gas composition; the first communication unit having a two-wire connection may be a microcomputer and the second communication unit having a single-wire connection part of the monitoring system.
It is known to control the fuel-to-air ratio of an internal combustion engine in response inter alia to the composition of the results of the exhaust gas with a view to reduction in air pollution, and frequently catalytic oxidation is adapted in order to substantially avoid the discharge into the atmosphere of carbon monoxide resulting from incomplete combustion of the fuel. To this end the exhaust gas is analysed by measurement of its thermal conductivity and its composition expressed as the percentage of carbon dioxide present. With microcomputer control of the fuel system there is a need for supplying to the microcomputer signals characterising the instantaneous actual composition and the corresponding desired composition of the exhaust gas as well as other engine performance parameters. The nature of these signals as regards their levels, input and output impedances and their frequency of transmission have been established by ISO specifications.
When the microcomputer is in a position to transmit data to a communications partner it is desirable that it should be able to do so without delay and a conventional two-wire interface involving direct connection between two wires can result in the computer receiving self-transmitted data which is undesirable for echo suppression.